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CONTROL OF corn earworms by pyrethroid insecticides such as Warrior, Baythroid, Capture and Mustang Max has decreased in efficacy in small plot trials by more than 50% over the past few years, according to studies by Richard Weinzierl, entomologist at the University of Illinois. This is unlikely to have a big impact with field corn growers.

“However, it is a big concern for seed corn or sweet corn growers,” Weinzierl says. “Those two groups could see significant impacts, because their available control options have just been reduced.”

Earworms can significantly affect the quality and quantity of seed corn, says Bill Hutchison, entomologist at the University of Minnesota Extension. “The corn earworm is a very aggressive insect and will feed on multiple kernels,” he says. “Given the high value of seed corn compared to field corn, farmers will want to protect as many kernels as possible from this type of late-season pest.”

Steep decline

In sweet corn research plots, Weinzierl, Hutchison and collaborators found that pyrethroid insecticide efficacy has declined from 98% control in the 1990s to 15 to 50% control now.

“The decline has been very rapid and widespread,” Weinzierl says. “We've seen instances in Illinois, Indiana, Wisconsin, Iowa and Minnesota. It can't be explained purely by timing or other procedural mistakes. And it's now been confirmed by insect bioassays. It's pyrethroid resistance.”

Weinzierl and others have noticed the trend for the last three to four years but say the decline has been rapid for a class of chemistry. Hutchison also emphasizes that “for several reasons, the loss of control in large, commercial sweet corn fields has fortunately been very limited to date; one reason may be that aerial applications also help control adult moths before they have a chance to lay eggs. The trends we are monitoring, however, do not bode well for the future.”

Migration from the South

“We never thought we would have this much of a problem in the Midwest, because the vast majority of corn acres aren't treated with pyrethroids,” Weinzierl notes. However, corn earworms do not overwinter in the upper Midwest; instead they migrate from southern states. And they are widely sprayed with pyrethroids in the South. “Our suspicion is that if there's a resistance problem, it stems from populations in the South,” Weinzierl says.

Earworm moths typically migrate during the late summer months, arriving from late July to mid-August in northern Illinois, and by mid-August to early September in northern Minnesota, Hutchison says. Before migrating north, the earworms have already been exposed to multiple pyrethroid applications on several southern crops, he says.

Although European corn borer Bt traits such as YieldGard ECB and Herculex I have activity on corn earworms, they don't provide the same level of control.

“They still have a high level of activity, but they don't provide the 98 to 99% control that farmers are used to with corn borer,” Hutchison says.

To facilitate discussion among researchers and the crop protection industry, a symposium will be held at the North Central Branch Entomology meetings in Bloomington, IL, March 27. For more information visit

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